GB2052838A - Superconducting cable - Google Patents

Description

1

SPECIFICATION

Superconducting cable The present invention relates to a superconducting cable comprising a plurality of wires, each having a plurality of filaments of superconducting material embedded in a matrix material, wires being laid- up together (e.g.

stranded or plaited) and soldered together to form wire bundles which are then stranded or plaited together to form ropes, which in turn are stranded or plaited together to form the cable.

Superconducting cables are used in particu- lar for the winding of coils which are intended for the excitation of very strong electromagnetic fields. According to the state of the art the cable comprises a plurality of ropes each made from wire bundles. Two processes are used to cool the cable down to the temperature necessary for the superconduction: bath cooling in which the whole coil is immersed in a bath of a cooling medium; and forced cooling in which a cooling medium is forced through the spaces between the wire bundles and the ropes (matrix cooling) and/or through cooling channels built into the cable (tubular conductor cooling). Cables intended to be matrix cooled are necessarily enclosed in a gas-tight case, while cables which are to be bath cooled have preferably no case.

On exciting magnetic coils, forces corresponding to the vectorial product of the excit- ing current and the magnetic induction act on the current conductors. These forces are directional and can cause a deformation of the conductors' cross-section and the windings' cross-section as well as a change in the rela- tive position of adjacent conductors. These deformations and changes of position can further cause a decrease in the contact pressure between neighbouring conductors and/ or a relative displacement of neighbouring conductors. Both phenomena are particularly 110 disadvantageous for a superconducting cable.

During the relative displacement of adjacent conductors heat can be generated from friction, which causes a small local rise in tem- perature and which is particularly disadvantageous at the operating temperature of superconducting cables.

The forces and the deformations they cause combine to a directional force on the inner wall of the enclosing casing, which can lead to an elastic deformation of the casing. For windings with tightly packed cable casings, the deformation forces of the cases are additive in the direction of the force, so that not only the associated Lorenz force but in addition the mechanically transmitted deformation of the casing acts on the individual cable.

In accordance with the present invention, there is provided a superconducting cable, comprising a plurality of wires each having a GB 2 052 838A 1 plurality of superconducting filaments embedded in a matrix material, the wires being laid-up and soldered to.gether to form wire bundles which are then stranded or plaited together to form ropes which in turn are stranded or plaited together to form the cable, the wire bundles of the rope being materially connected together or to neighbouring ropes in the area of their points or lines of contact in order to increase the mechanical strength and improve the thermal and electrical conductivity as well as for a more effective matrix cooling.

Thus, in the superconducting cable, the individual conductors (i.e. bundles) are only elastically and not plastically deformed by the electro-magnetic forces and also cannot be displaced relative to one another.

Preferably the bundle of wires in the rope and the adjacent ropes are materially connected (or bonded) together in the region of the points or lines of contact.

With the materially connecting join of the individual conductors in this superconducting cable, it can be effectively prevented that the adjacent conductors change their relative positions and thus that troublesome frictional heat arises. Also the thermal resistance is not changed on exciting the coil, whereby an optimal cooling and the rapid equalising of thermal instabilities are assured. Finally the materially connecting join forms a self-supporting mechanical construction, which can absorb a large fraction of the forces created by the electromagnetic field.

One preferred form of materially connecting join comprises soft soldering.

In a preferred embodiment, the bundles of wires are arranged in several layers on top of each other to form a lattice, with the bundles in each layer aligned parallel to each other, whereby the adjacent layers of wire bundles together form an angle whose bisector lies along the longitudinal direction of the cable.

Embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a super- conducting wire; Figure 2 is a plan view of a superconducting wire, which is wound with a material of high electrical resistivity; Figure 3 is a plan view of a superconducting wire, which is encased in a material of high electrical resistivity; Figure 4 is a perspective view of a wire bundle containing a plurality of superconducting wires and stabilising wires stranded to- gether; Figure 5 is a perspective view of a plurality of wire bundles plaited in a lattice formation to form a rope; Figure 6 is a cross-section through a rope encased in a material of high electrical resis- 2 GB2052838A 2 tivity, the rope including several plaited wire bundles; Figure Y is a cross-section through a rope encased in a material of high electrical resistivity, the rope comprising a plurality of wire bundles wound with a material of high electrical resistivity and then stranded together; Figure 8 is a cross-section through a rope, which comprises several bundles of wires stranded together in coaxial layers; Figure 9 is a schematic cross-section through a cable, which comprises a plurality of plaited ropes each composed of several wire bundles; Figure 10 is a cross-section through a cable 80 which includes a gas-tight casing and is in tended for forced cooling; Figure 1 Oa shows the longitudinal profile, seen along the section A-A of Fig. 10, of a heat insulator which is inserted between the rope bundles and the casing of the cable of Fig. 10; Figure 11 is a part cross-section of a coil winding, intended to be immersed in a cool- ing bath; and Figure 12 is a schematic representation of an apparatus for manufacturing a supercon ducting cable in accordance with the present invention.

Fig. 1 shows a superconducting wire con sisting of a plurality of superconducting fila ments 10, which for thermal and electrical stability are loosely stranded together and embedded in a matrix 11 of an electrically and thermally good conducting metal. The wire is coated with a thin layer 12 of soft solder, which makes possible the production of soldered joints to other, adjacent wires.

Fig. 2 is a plan view of a superconducting wire 14, which is spirally wound with a strip 15 of a heat conducting material with high electrical resistivity. As already mentioned, this winding is intended to reduce as far as possible the unwanted eddy- currents between neighbouring wires. The strip 15 is preferably soldered to the wire 14 over the total area of contact. For the example shown, the strip is wound such that neighbouring side edges have a gap between them and a spiral shaped groove 16 is formed between the individual coils. This groove can be used as a channel for coolant for cables with matrix cooling.

It is also possible to wind the strip such that its side edges lie close together or with overlapping edges, should this be simpler or more advantageous.

It is further possible, instead of the strip 15 shown, to wind a wire of heat conducting material of high electrical resistivity around the superconducting wire and then solder it, whereby the individual windings of the wire can be chosen to have a gap between them or lie close together, or even be wound with a variable gap.

A suitable heat conducting material with 130 high electrical resistivity is a conventional copper-nickel alloy.

In Fig. 3, the superconducting wire 18 is surrounded by a soldered-on casing 19 of a -thermally conducting material with a high electrical resistivity. The casing has openings 20, 21 which are distributed around the whole circumference and arranged at regular intervals and which are provided for the direct influx of the coolant to the superconducting wire.

Fig. 4 shows a wire bundle composed of a plurality of superconducting wires 50 which are stranded with normally conducting wires 51,52 of different diameters, which act as stabilisers. These stabilising wires are made for example out of copper or aluminium. All wires are coated with a thin surface layer of solder and in the regions of the lines of contact are soldered together. The superconducting wires may be as shown in Fig. 1,2 or 3 for example.

The stranded wire bundle is surrounded by a perforated casing 54 of a thermally conduct- ing material with a high electrical resistivity, in order to avoid unwanted eddy-currents in neighbouring wire bundles.

Fig. 5 shows a rope, made in the form of a spatial lattice. This rope is composed of a plurality of wire bundles 23 to 37. The wire bundles form grid layers, whereby the individual layers are made from wire bundles 23,24,25; or 26, 27; or 28,29,30; or 31,32; or 33,34,35 or 36,37 with the bundles in each layer arranged parallel to one another. The wire bundles of adjacent layers lying on top of one another occlude an angle 39, the bisector of which lies parallel to the longitudinal direction of the cable, marked with an arrow 40, along which the current will flow. The wire bundles are soldered together in the region of all contact areas as is shown with the marked solder beads for example 42,43,44,45,46.

The current through the rope is practically uniformly distributed among the individual wire bundles. If one of the wire bundles is broken or if a part of a wire bundle jumps from the superconducting to normally conducting condition, for example because of a thermal instability, then the neighbouring wire bundles take over the current conduction. This is shown in Fig. 5 with the example of the wire bundle 29, which has a nonconducting or non-superconducting part 48. The current in the wire bundle 29 flows then in the wire bundles 31 and 26, where it is further passed on into at least the wire bundles 34 and 30 and 24 and 30 respectively. It should thereby be understood, that on disruption of a wire bundle, practically all other wire bundles of the rope take over a part of the current conduction, which does not need to be described in detail here. It can also be clearly recognised from Fig. 5 that the solder beads 1 GB 2 052 838A 1 SPECIFICATION

Superconducting cable The present invention relates to a superconducting cable comprising a plurality of wires, each having a plurality of filaments of superconducting material embedded in a matrix material, wires being laid- up together (e.g, stranded or plaited) and soldered together to form wire bundles which are then stranded or plaited together to form ropes, which in turn are stranded or plaited together to form the cable Superconducting cables are used in particu- lar for the winding of coils which are intended for the excitation of very strong electromagnetic fields. According to the state of the art the cable comprises a plurality of ropes each made from wire bundles. Two processes are used to cool the cable down to the temperature necessary for the superconduction: bath cooling in which the whole coil is immersed in a bath of a cooling medium; and forced cooling in which a cooling medium is forced through the spaces between the wire bundles and the ropes (matrix cooling) and/or through cooling channels built into the cable (tubular conductor cooling). Cables intended to be matrix cooled are necessarily enclosed in a gas-tight case, while cables which are to be bath cooled have preferably no case.

On exciting magnetic coils, forces corresponding to the vectorial product of the excit- ing current and the magnetic induction act on the current conductors. These forces are directional and can cause a deformation of the conductors' cross-section and the windings' cross- section as well as a change in the rela- tive position of adjacent conductors. These deformations and changes of position can further cause a decrease in the contact pressure between neighbouring conductors and/ or a relative displacement of neighbouring conductors. Both phenomena are particularly disadvantageous for a superconducting cable.

During the relative displacement of adjacent conductors heat can be generated from friction, which causes a small local rise in tem- perature and which is particularly disadvantageous at the operating temperature of superconducting cables.

The forces and the deformations they cause combine to a directional force on the inner wall of the enclosing casing, which can lead to an elastic deformation of the casing. For windings with tightly packed cable casings, the deformation forces of the cases are additive in the direction of the force, so that not only the associated Lorenz force but in addition the mechanically transmitted deformation of the casing acts on the individual cable.

In accordance with the present invention, there is provided a superconducting cable, comprising a plurality of wires each having a plurality of superconducting filaments embedded in a matrix material, thewires being laid-up and soldered together to form wire bundles which are then stranded or plaited together to form ropes which in turn are stranded or plaited together to form the cable, the wire bundles of the rope being materially connected together or to neighbouring ropes in the area of their points or lines of contact in order to increase the mechanical strength and improve the thermal and electrical conductivity as well as for a more effective matrix cooling.

Thus, in the superconducting cable, the individual conductors (i.e. bundles) are only elastically and not plastically deformed by the electro-magnetic forces and also cannot be displaced relative to one another.

Preferably the bundle of wires in the rope and the adjacent ropes are materially connected (or bonded) together in the region of the points or lines of contact.

With the materially connecting join of the individual conductors in this superconducting cable, it can be effectively prevented that the adjacent conductors change their relative positions and thus that troublesome frictional heat arises. Also the thermal resistance is not changed on exciting the coil, whereby an optimal cooling and the rapid equalising of thermal instabilities are assured. Finally the materially connecting join forms a self-supporting mechanical construction, which can absorb a large fraction of the forces created by the electromagnetic field.

One preferred form of materially connecting join comprises soft soldering.

In a preferred embodiment, the bundles of wires are arranged in several layers on top of each other to form a lattice, with the bundles in each layer aligned parallel to each other, whereby the adjacent layers of wire bundles together form an angle whose bisector lies along the longitudinal direction of the cable.

Embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a super- conducting wire; Figure 2 is a plan view of a superconducting wire, which is wound with a material of high electrical resistivity; Figure 3 is a plan view of a superconducting wire, which is encased in a material of high electrical resistivity; Figure 4 is a perspective view of a wire bundle containing a plurality of superconducting wires and stabilising wires stranded to- gether; Figure 5 is a perspective view of a plurality of wire bundles plaited in a lattice formation to form a rope; Figure 6 is a cross-section through a rope encased in a material of high electrical resis- 2 GB2052838A 2 tivity, the rope including several plaited wire bundles; Figure 7 is a cross-section through a rope encased in a material of high electrical resistivity, the rope comprising a plurality of wire bundles wound with a material of high electrical resistivity and then stranded together; Figure 8 is a cross-section through a rope, which comprises several bundles of wires stranded together in coaxial layers; Figure 9 is a schematic cross-section through a cable, which comprises a plurality of plaited ropes each composed of several wire bundles; Figure 10 is a cross-section through a cable 80 which includes a gas-tight casing and is in tended for forced cooling; Figure 10a shows the longitudinal profile, seen along the section A-A of Fig. 10, of a heat insulator which is inserted between the rope bundles and the casing of the cable of Fig. 10; Figure 11 is a part cross-section of a coil winding, intended to be immersed in a cool- ing bath; and Figure 12 is a schematic representation of an apparatus for manufacturing a supercon ducting cable in accordance with the present invention.

Fig. 1 shows a superconducting wire con sisting of a plurality of superconducting fila ments 10, which for thermal and electrical stability are loosely stranded together and embedded in a matrix 11 of an electrically and thermally good conducting metal. The wire is coated with a thin layer 12 of soft solder, which makes possible the production of soldered joints to other, adjacent wires.

Fig. 2 is a plan view of a superconducting wire 14, which is spirally wound with a strip of a heat conducting material with high electrical resistivity. As already mentioned, this winding is intended to reduce as far as possible the unwanted eddy-currents between neighbouring wires. The strip 15 is preferably 110 The current through the rope is practically soldered to the wire 14 over the total area of uniformly distributed among the individual contact. For the example shown, the strip is wire bundles. If one of the wire bundles is wound such that neighbouring side edges broken or if a part of a wire bundle jumps have a gap between them and a spiral shaped from the superconducting to normally con- groove 16 is formed between the individual 115 ducting condition, for example because of a coils. This groove can be used as a channel thermal instability, then the neighbouring wire for coolant for cables with matrix cooling. bundles take over the current conduction. This It is also possible to wind the strip such that is shown in Fig. 5 with the example of the its side edges lie close together or with over- wire bundle 29, which has a non-conducting lapping edges, should this be simpler or more 120 or non-superconducting part 48. The current advantageous. in the wire bundle 29 flows then in the wire It is further possible, instead of the strip 15 bundles 31 and 26, where it is further passed shown, to wind a wire of heat conducting on into at least the wire bundles 34 and 30 material of high electrical resistivity around and 24 and 30 respectively. It should thereby the superconducting wire and then solder it, 125 be understood, that on disruption of a wire whereby the individual windings of the wire bundle, practically all other wire bundles of can be chosen to have a gap between them or the rope take over a part of the current lie close together, or even be wound with a conduction, which does not need to be de variable gap. scribed in detail here. It can also be clearly A suitable heat conducting material with 130 recognised from Fig. 5 that the solder beads high electrical resistivity is a conventional copper-nickel alloy.

In Fig. 3, the superconducting wire 18 is surrounded by a soldered-on casing 19 of a thermally conducting material with a high electrical resistivity. The casing has openings 20, 21 which are distributed around the whole circumference and arranged at regular intervals and which are provided for the direct influx of the coolant to the superconducting wire.

Fig. 4 shows a wire bundle composed of a plurality of superconducting wires 50 which are stranded with normally conducting wires 51,52 of different diameters, which act as stabilisers. These stabilising wires are made for example out of copper or aluminium. All wires are coated with a thin surface layer of solder and in the regions of the lines of contact are soldered together. The superconducting wires may be as shown in Fig. 1,2 or 3 for example.

The stranded wire bundle is surrounded by a perforated casing 54 of a thermally conduct- ing material with a high electrical resistivity, in order to avoid unwanted eddy-currents in neighbouring wire bundles.

Fig. 5 shows a rope, made in the form of a spatial lattice. This rope is composed of a plurality of wire bundles 23 to 37. The wire bundles form grid layers, whereby the individual layers are made from wire bundles 23,24,25; or 26, 27; or 28,29,30; or 31,32; or 33,34,35 or 36,37 with the bundles in each layer arranged parallel to one another. The wire bundles of adjacent layers lying on top of one another occlude an angle 39, the bisector of which lies parallel to the longitudinal direction of the cable, marked with an arrow-40, along which the current will flow. The wire bundles are soldered together in the region of all contact areas as is shown with the marked solder beads for example 42,43,44,45,46.

v 3 GB2052838A 3 between neighbouring bundles of wires do not decrease the free space in the lattice structure and thereby do not restrict the flow of the coolant.

It is to be understood that instead of the rope shown in Fig. 5 which is made simply from wire bundles lying on top of one another, ropes of wire bundles plaited together into a circular cross-section are prefera- bly used, as is shown in Fig. 8 which is yet to be described.

Fig. 6 shows schematically a cross-section through a cable rope which contains eight wire bundles, of which only the bundle 55 is identified with a reference number. Each wire bundle is preferably made from superconducting and stabilising wires stranded together as shown for example in Fig. 4, or from plaited and transposed wires, whose position in the middle and outside respectively of the rope is periodically interchanged. This last arrangement makes possible a better suppression of unwanted induced currents. The rope has a winding or surrounding 56 of a thermally conducting material with high electrical resistivity. The individual wire bundles are soldered together and to the winding or easing at the points or lines of contact. It is to be understood that the casing has many open- ings, so that the coolant can as far as possible flow unrestricted into the rope and past the wire bundles.

Fig. 7 shows schematically the section through another embodiment of a rope. This contains six wire bundles of which only the wire bundle 57 is identified with a reference number. Each wire bundle comprises a plurality of wires stranded together as shown in Fig. 4, and is wound with a strip 58 of a thermally conducting material with a high electrical resistivity, as shown in Fig. 2 for the wire 2. The winding is done such that the width of the groove between neighbouring windings is greater than the width of the strip itself, and the windings of neighbouring wire bundles can be -screwed into- each other. The wire bundles are stranded around a central stabilising wire 59 and encased in a surround 60. For this rope the points or lines of contact between the individual wire bundles and the wound around strip and the strip and the casing are soldered together. As can be recognised from the drawing, the cross-section of this rope has much free space, which is well suited or the transmission and distribution of a cooling medium.

Fig. 8 is the section through yet another embodiment of rope. The rope contains a relatively thick core-wire 61 of normally conducting copper or aluminium, around which three layers 62,63,64 of wire bundles are coaxially arranged. _Each layer is stranded from a plurality of wire bundles, e.g. the wire bundles 65 or 66; 66 or 67. The layers are stranded with alternating direction of rotation, as indicated by the arrows 68,69,70. Many contact points thus arise, at which the individual wire bundles can be soldered together, whereby a relatively rigid cable structure is formed, which holds the wire bundles and the rope layers firmly in their predetermined positions.

Each of the coaxially arranged rope layers can be surround by a perforated thermally conducting material with high electrical resistivity. The spaces between the individual rope layers and between the wire bundles can be used as cooling channels for matrix cooling.

Fig. 9 shows schematically the section through an embodiment of cable, in which ropes 71, each of which comprises a plurality of wire bundles (not shown), are plaited together for an optimum transposition, so that the location of the ropes in the cable cross- section is periodically changed. The ropes are soldered together in the region of the points or lines of contact.

Fig. 10 is the section through a cable which includes an enclosing casing. The cable con- tains a plurality of outer ropes 73 to 77 which are stranded around or plaited to the central rope 78. Each rope is composed of several wire bundles, which at the contact points or lines are soldered together and to a casing 79 which encloses the ropes. The casings 79 of the ropes are likewise soldered together in the region of the points or areas of contact. The ropes are placed into the cable casing, which is welded from two U-shaped shells 80,81.

These cable casings from the outer limits of the cooling channels for matrix cooling, and can, for example when they are made from steel, absorb external forces and relieve the enclosed parts of the superconducting cable.

On welding the two shells together such that the joint abutment is vacuum tight, a very high temperature arises in the region of the welding beads 82,83. In order to protect the enclosed superconductors from this very high temperature, the ropes are preferably bandaged with a poor thermally conducting profiled steel strip 84. Fig. 10a shows the seetion through the steel strip along the line A-A in Fig. 10.

Fig. 11 is a section through a part of a coil wound with a superconducting cable, intended for bath cooling. The winding comprises a plurality of superconducting cables 86, constructed corresponding to the cable shown in Fig. 9. The coil contains a coil former with coil insulation 87 and winding insulations 88, between which the cable windings are inserted. The coil and winding insulation separate the cable windings electri- cally from one another and form at the same time an effective mechanical framework for the whole coil.

The insulations are provided with large openings 89 and 90, which are to allow as far as possible the unrestricted flow of coolant 4 GB2052838A 4 through the coil.

An apparatus for the construction of the cable in accordance with the present invention is shown schematically in Fig. 12. The apparatus has an inlet station 93 with two rollpairs set at 90' to each other. The pressure of the roll-pairs directed along the cable axis causes a deformation and compacting of the cable cross-section and especially the points and areas of contact, whereby the intended joining points or lines are enlarged by plastic flow. The cable then moves into a wetting station 94, in which it is impregnated with a flux, which facilitates the subsequent solder- ing. Afterwards the cable is fed through a soldering and calibrating station 95. A heating set-up 97 is arranged at the entrance to this station, which heats the whole cable electrically or inductively to a temperature which lies above the melting point of the solder. Two repressing roll-pairs mounted perpendicular to each other are arranged in the feed direction of the cable after the heating set-up, in which the cable is again pressed together, whereby the liquid solder forms thin layers of solder at the contacting parts of the wire bundles and the ropes. A cooling set-up 99 and two pairs of sizing rolls 100 arranged perpendicular to each otherform the exit from the soldering and calibrating station. As the cable goes through the sizing rolls, the cable cross-section is pressed together to the required final dimension and at the same time cooled to a temperature at which the solder solidifies. A washing station 102 is provided after the soldering and calibrating station. In this station the cable is sprayed with a cleansing agent which washes away any remains of the flux and other contaminants.

The apparatus described makes possible the soldering of a prepared cable in a continuous process, and the soldered and cross-sectioncalibrated cable is then ready to be enclosed in a casing or for bath cooling to be wound round a coil core.

Suitable solders for the process described are for example lead-tin or tin-silver solders.

It is to be understood that the materially connected join can equally well be produced by welding or by a diffusion-bonding process.

On drawing the cable through the apparatus described above and pressing the cable crosssection in the entry roll pair or re-pressing roll pair, the parts of the cable are plastically formed. Furthermore, on laying the cable in the casing, the parts of the cable are elastically pre-stressed. It is thus achieved that, under the action of the electro-magnetic forces, the soldered cable parts can only be deformed elastically in the range of this prestress, and the deformation energy transformed into heat in minimised.

Claims (10)

1. A superconducting cable, comprising a plurality of wires each having a plurality of superconducting filaments embedded in a matrix material, the wires being laid-up and soldered together to form wire bundles which are then stranded or plaited together to form ropes which in turn are stranded or plaited together to form the cable, the wire bundles of the rope being materially connected together or to neighbouring ropes in the area of their points or lines of contact in order to increase the mechanical strength and improve the thermal and electrical conductivity as well as for a more effective matrix cooling.

2. A cable according to claim 1, in which the wire bundles and/or the ropes incorporate stabilising elements which are materially connected together and to neighbouring wire bundles in the area of the points or lines of contact.

3. A cable according to claim 1, in which the wires and/or the wire bundles and/or the ropes are at least partly surrounded by a nonmagnetic thermally conducting material with high electrical conductivity in order to sup- press eddy currents on the production of or application in alternating magnetic fields, and are materially connected to this surround and to neighbouring surrounds in the areas of the points or lines of contact.

4. A cable according to claim 1,2 or 3, in which the materially connecting join comprises soldering.

5. A cable according to any preceding claim, in which the wire bundles are arranged lattice-like in several layers on top of each other and aligned parallel to each other within each layer, whereby the wire bundles of neighbouring layers occlude an angle whose bisector lies along the longitudinal direction of the cable.

6. A cable according to claim 5, in which the wire bundles are plaited lattice-like in several layers.

7. A cable according to claim 1, in which the cable comprises a casing enclosing the stranded or plaited ropes.

8. A process of manufacturing a cable according to claim 1, in which the stranded or plaited ropes of stranded or plaited wire bundles are pre-compressed to the required cross-section with the wires coated with a thin layer of solder, impregnated with a flux and then heated above the melting point of the solder, and deformed and compressed during cooling to the final cross-section and finally the surplus flux is washed off.

9. A process of manufacturing a cable, which process is as claimed in claim 8 and substantially as herein described.

10. A superconducting cable, substantially as herein described with reference to Figs. 1 to 11 of the accompanying drawings.

1 GB 2 052 838A 5 Printed for Her Majesty's Stationery Office by Burgess Et Son (Abingdon) Ltd.-I 98 1. Published at The Patent Office, 25 Southampton Buildings, London, WC2A IAY, from which copies may be obtained.